In FRTL-5 cells, thyroid stimulating hormone (1x10-11M) increases both malic enzyme mRNAs (21S and 27S) about 7-8 fold, 16 hours after the hormone addition. The cells are in the G phase. Since the rate of transcription was unchanged during the TSH incubation as determined by nuclear run on assay and since cyclo-heximide abolished the TSH effect, protein(s) are probably involved in the mechanism by which TSH increases the malic enzyme mRNA level via cAMP. We have continued to study nuclear mechanisms by which thyroid hormone regulates malic enzyme (ME) mRNA in liver cytoplasm. The study included two ME intronic probes and nearly full size ME cDNA. These probes were used in nuclear run off and slot blot analyses of nuclear RNA preparations from euthyroid and T3 treated rat liver. We conclude that T3 activates transcription of the malic enzyme gene in rat liver and decreases the rate of degradation of pre-mRNA coding for malic enzyme. As a control, we examined the effect of a high carbohydrate diet which is known to increase malic enzyme mRNA without affecting either transcriptional rate or nuclear RNA (Proc Natl Acad Sci USA 83:4705). As expected, no change in the level of malic enzyme RNA in the nucleus was found with the intronic probes.